Union. Given two sets A and B, A ∪ B consists of the set containing all elements in either A or B. Another simplistic example:

A = { 1, 3, 5, 7 }
B = { 2, 4, 6, 8 }

A ∪ B = { 1, 2, 3, 4, 5, 6, 7, 8 }

∪ is written in HTML as ∪
See also:

Although not illustrated above, when forming the union of two sets, elements that appear in both original sets are only included once in the union set, eg.

{1, 3, 5, 7} ∪ {1, 3, 6}
= {1, 3, 5, 6, 7}

I believe if you wish to include the duplicated elements twice, you need to give them a subscript in the source sets. This doesn't affect the value of the number itself but the elements are technically different and so should be included twice, eg.

{21, 3} ∪ {1, 22}
={1, 21, 22, 3}

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